The present invention generally relates to an improved method and apparatus for on-line cleaning of deposited matter from the surfaces of the heat-sink media in Regenerative Fume Incinerators (RFIs). Specifically, it covers a system for washing down the deposited matter from the heat transfer surfaces of the heat sink media (HSM) within a Regenerative Heat Exchanger (RHX) of a regenerative fume incinerator.
Regenerative fume incinerators are widely used in industry to clean polluted gas streams containing combustible pollutants before the gas stream is exhausted to the atmosphere. As used herein, the term “Regenerative Fume Incinerator” includes Regenerative Thermal Oxidizers (RTOs), Regenerative Catalytic Oxidizers (RCO) and Thermal Catalytic Oxidizers (TCO).
Regenerative thermal oxidizers, regenerative catalytic oxidizers, and thermal catalytic oxidizers use different oxidation processes to destroy the pollutants in the polluted gas stream. As defined herein, a regenerative thermal oxidizer maintains a high operating temperature (between 1,200 to 2,000 degrees F.) in the combustion chamber to facilitate the oxidation of the pollutants in the polluted gas stream. Regenerative thermal oxidizers have been well described in the prior art such as U.S. Pat. No. 5,098,286 to York, U.S. Pat. No. 5,259,757 to Plejdrup et al. and others. Briefly, a regenerative thermal oxidizer generally comprises a combustion chamber in fluid communication with a plurality of regenerative heat exchangers. The polluted gas is first passed through a previously heated regenerative heat exchanger and is preheated to a high temperature. The preheated polluted gas is then passed into the combustion chamber where it is further heated to a temperature high enough for generally complete oxidation of the combustible pollutants to a cleansed gas containing harmless end-products such as water and carbon-dioxide. The cleansed hot gas is then passed into a second regenerative heat exchanger which was previously cooled by the passage of the cold polluted gas through it. The cleansed hot gas releases its sensible heat to the relatively cooler heat sink media in the second regenerative heat exchanger which gets heated for use in a subsequent preheating cycle as described above. The cold polluted gas and cleansed hot gas are alternately passed through the two regenerative heat exchangers to maintain continuity of flow and heat transfer between the cold and hot gas streams.
As is well known and practiced in the art, more than two regenerative heat exchangers can be used for increased capacity and to enhance the pollutant destruction capability of the regenerative fume incinerator. An regenerative thermal oxidizer with more than two regenerative heat exchangers, which uses a purge system to recycle entrapped polluted gas, is described in the aforementioned patent to York.
A regenerative catalytic oxidizer is defined herein as a regenerative fume-incinerator that is designed similar to a regenerative thermal oxidizer. However, it includes a catalyst to facilitate the oxidation of the pollutants in the polluted gas stream at a relatively lower temperature (about 400 to 800 degrees F.) to save energy.
A thermal catalytic oxidizer is defined herein as a regenerative fume-incinerator which is a hybrid regenerative catalytic oxidizer and regenerative thermal oxidizer. A thermal catalytic oxidizer is designed to operate initially at a relatively lower oxidizing temperature (about 400 to 800 degrees F.) using a catalyst (as in a regenerative catalytic oxidizer) and to operate at a high oxidizing temperature (about 1,200 to 2,000 degrees F. as in a regenerative thermal oxidizer) after the catalyst is deactivated. This feature provides operating flexibility.
It is well known in the art that the relatively densely packed heat sink media in the regenerative heat exchangers of regenerative fume incinerators is quite susceptible to fouling due to the deposition of condensable and non condensable aerosols in the polluted air streams. Since the fouling tends to vitiate the performance of the regenerative fume incinerator, techniques have been developed to clean the heat sink media in fouled regenerative heat exchangers. For example, Plejdrup et al. describe a method of cleaning the condensed combustible matter from the heat transfer surfaces of the heat sink media in a regenerative thermal oxidizer by passing the hot oxidized gas through a fouled regenerative heat exchanger bed for an extended period of time. However, while this “burn-out” (also referred to as “bake-out”) method is useful for removing combustible deposited matter, it is not very useful in removing non-combustible deposited matter from the regenerative heat exchanger.
U.S. Pat. No. 6,579,379 to Noble describes a method (the Noble method) and apparatus to remove deposited matter from the surfaces of the heat sink media in a regenerative heat exchanger. However, the Noble method suffers from various disadvantages, the primary one of which is that it is mostly manual in nature. In the Noble method, the regenerative fume incinerator has to be shutdown and cooled to ambient temperature before the cleaning apparatus is manually assembled and operated within the regenerative fume incinerator. The shut-down and cooling requirement results in an interruption of production for a fairly long period of time. The Noble method also requires additional time to manually assemble and disassemble the cleaning apparatus in the regenerative fume incinerator. These time requirements result in lost revenue and profits for the regenerative fume incinerator user. Further, the Noble method is not effective against sticky combustible deposited matter which cannot be easily dissolved by a water wash. Therefore, a burn-out operation is required to gasify the sticky combustible deposited matter prior to the wash-down. Cooling the regenerative heat exchanger bed from the higher burn-out temperature requires additional time which further increases loss of production.
As a particular example, regenerative fume incinerators used in the wood industry are subjected to fouling by fine wood particles as well as sticky condensable combustible resin particles. This is a particularly difficult fouling situation which requires that the regenerative heat exchanger be first subjected to a burn-out operation to remove the combustible deposited matter and then washed out to remove the residual non-combustible deposited matter such as inorganic salts which are present in wood particles. The Noble method is not particularly well suited to this application because, during the burn-out operation, the temperature of the heat sink media in the regenerative heat exchanger is raised to a higher level than normal to effect gasification of the combustible matter. Therefore, the regenerative fume incinerator takes a much longer time to cool to ambient temperature as required in the Noble method. Further, the Noble method requires operating personnel to open the regenerative fume incinerator and enter into a potentially hazardous confined area, thereby potentially jeopardizing the lives of the personnel. Yet further, the Noble method requires that all beds be cleaned during a cleaning operation. The Noble method does not disclose a way to selectively clean one or more of the regenerative heat exchangers in a regenerative fume incinerator as needed due to adverse fouling conditions associated with these regenerative heat exchangers.
There is therefore a need for a method and apparatus to burn-out and wash-down a regenerative heat exchanger bed while the regenerative fume incinerator is on-line with the process. The method has to be able to quickly and efficiently clean the regenerative heat exchanger bed without shutting down the regenerative fume incinerator and without cooling the regenerative fume incinerator to ambient temperature. The method should be safe to practice and should not require the entry of personnel into a hazardous confined area. Further, the method should be able to selectively clean-out one or more of the regenerative heat exchangers of a regenerative fume incinerator without shutting down the regenerative fume incinerator.